The strobilurin fungicide methyl (E)-2-{2-[6-(2-cyanophenoxy)pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate (azoxystrobin), known by the common name azoxystrobin, is a widely used commercial agrochemical product. It is described in “The Pesticide Manual” published by the British Crop Protection Council, 12th edition, pp 54-55 and in the proceedings of the Brighton Crop Protection Conference (Pests and Diseases) 1992, Volume 1, 5-6, pp 435-442. It was first disclosed in EP-A-0382375 (compound 9, Example 3) along with methods for its preparation.
Azoxystrobin was first marketed in 1998 and is a systemic, broad-spectrum fungicide with activity against the four major groups of plant pathogenic fungi including Ascomcetes (e.g., powdery mildews), Basidiomycetes (eg rusts), Deutoromycetes (e.g., rice blast) and Oomycetes (e.g., downy mildew). It inhibits spore germination and mycelial growth. It has worldwide uses on cereals, vines, rice, citrus, potatoes and tomatoes. In 1999, azoxystrobin was the leading proprietary fungicide worldwide and is now a world market leader in cereals.
There are several reported ways of making azoxystrobin. Several methods are based on the construction of the methyl α-phenyl-β-methoxyacrylate group at an early stage followed by building the central pyrimidinyloxy and terminal cyanophenoxy rings. For example, (E)-methyl 2-(2-hydroxyphenyl)-3-methoxyacrylate may be reacted with 4,6-dichloropyrimidine under alkaline conditions in N,N-dimethylformamide to form (E)-methyl 2-[2-(6-chloropyrimidin-4-yloxy)phenyl)-3-methoxyacrylate which is then reacted with 2-cyanophenol in an Ullmann-type coupling process (see EP-A-0382375). The (E)-methyl 2-(2-hydroxyphenyl)-3-methoxyacrylate may be prepared by the formylation and subsequent methylation of methyl 2-benzyloxyphenylacetate followed by removal of the benzyl protecting group (see EP-A-0242081). Formylation and methylation techniques for preparing the methyl α-phenyl-β-methoxyacrylate group are also described in WO 97/30020 and WO 97/01538.
U.S. Pat. No. 7,084,272 discloses an alternate method for preparing azoxystrobin by constructing the methyl-α-phenyl-β-methoxyacrylate group after building on the central pyrimidinyloxy ring or the central pyrimidinyloxy ring and terminal cyanophenol ring. Reportedly, this avoids a Smiles-type rearrangement and delivers the desired E-isomer.
There is an urgent and unmet need in the art for efficient methods for the preparation and purification of azoxystrobin, which are simple and can be used on a large scale for industrial manufacture, and which produce highly pure product that can be safely utilized.